How to access node tree of a selected node group in python?
First off, to get the currently active node, you can use 

currNode = context.active_node


from within an addon / Operator (which I suppose you're doing, otherwise there would be no bpy.context.selected_nodes). From the Python Console, getting the currently active node in the compositing tree would be

currNode = bpy.context.scene.node_tree.nodes.active


From there, you can use

currNode.id_data


to retrieve the tree this node lives in. This also works in Material Node Trees. To retrieve the materials (remember it could theoretical be more than one material using the same node tree) you can to:

print ([x for x in D.materials if x.node_tree == currNode.id_data])


So in your example:

# get a node first
x = D.node_groups['NodeGroup'].nodes['Group Input']

# now get the tree of this node (which is the node group)
x.id_data    # this gives you 'Node Group' again


Edit: Find the node tree of a group

After the question has been rephrased, this solution would be smarter: Node groups simply have an own Node Tree themselves. You can think of it like:

ShaderNodeTree.nodes[someGroup].node_tree.nodes[someOtherGroup].node_tree.nodes.thisCanGoOnForever


So in your case:

active_node = context.active_node
# active_node.name: Group.003
# active_node.group: Test 1

# find an image node in the group node tree
# groups have their own node tree to collect nodes
img_node = active_node.node_tree.nodes.get("Image Texture 1")


If the Image Texture named "Image Texture 1" exists, img_node will be the object. Otherwise, img_node will be None.